I did 23andme, and so did our mystery man. Two of my grandfather's brothers also did 23andme, as well as a number of the kids and grandkids of my grandfather's brothers and sisters (my grandfather had 9 siblings).
23and me shows this man as my dad's cousin (my first cousin once removed), and the man's number (8.00% DNA shared) is entirely outside the range an uncle would be per their website (17%-34%), and entirely inside the range for cousins once removed (2% to 11.5%). This man's number is consistent with many of my other known first cousins once removed on 23andme (all in the 6 to 8% range). So far, so good.
But this man and my grandfather (now deceased) both did Ancestry.com (my grandfather did it many, many years ago). That website shows this man to be 100% certain to be the son of my grandfather, so my uncle (my dad's brother), with 3480 cM across 25 segments.
There is the question. One website seems to rule my grandfather out as this man's father, the other seems to rule him in. There is a conflict, and I don't understand how this can be.